1. Field of the Invention
The present invention relates generally to semiconductor device fabrication and, more particularly, to a method of mitigating effects of copper oxidation.
2. Description of Related Art
Copper is an attractive metal for use in integrated circuits because of its relatively low cost and relatively high conductivity. Its nearest competitor in the metal conductor category may be silver. While silver is a slightly better conductor than copper, its price is significantly higher. To appreciate the relationship, if the conductivity of copper is normalized to one (1.0), then the corresponding normalized conductivity of silver is about 1.06, about 6 percent higher (better) than that of copper. If the price of copper is normalized to one, however, then the normalized price of silver is about 120. For about the same conductivity, silver costs 120 times (two orders of magnitude) more. That is, using silver rather than copper in an integrated circuit trades a 6 percent improvement in conductivity for what may be about a 1.2×104 percent increase in material cost.
While presenting a substantial cost advantage, copper suffers from a significant disadvantage because of a tendency of copper to oxidize when exposed to air, thus forming a copper oxide coating on copper surfaces. Because copper oxide is nonconductive, methods of manufacturing integrated circuits that employ copper as a conductor necessarily must take steps to address the problem of oxidation of copper surfaces. Furthermore, a total amount of highly-conductive copper in a structure, such as a copper wire, layer or lead, is reduced from oxidation of a surface thereof, naturally, as a consequence of copper molecules of the surface being converted, e.g., into copper oxide, the reduced volume thus tending to detract from the total conducting capacity of the structure.
A need thus exists in the prior art for a method of preventing copper oxidation in integrated circuit fabrication. A further need exists for a method of mitigating effects of copper oxidation when it does occur.